Systems have been developed to activate and detect remote activatable marker assemblies positioned, for example, in or on a selected object or body. The markers generate a signal used to detect the presence of the marker. Many of the activatable markers are hard-wired to a power source or other equipment external from the object. Other systems have been developed that utilize resonating leadless or “wireless” markers. These wireless markers are typically activated or energized by a remote excitation source that wirelessly transmits a strong continuous or pulsed excitation signal. In response to the excitation signal, the wireless markers wirelessly transmit a detectable marker signal that must be distinguished from the strong excitation signal and then analyzed in an effort to try to accurately determine the location of the target. The process of distinguishing a weak marker signal from the strong excitation signal to consistently and accurately determine the location of the marker has proven to be very difficult.
One example is U.S. Pat. No. 5,325,873 to Hirschi et al., which teaches a system that detects the general position of an object within a body of tissue. The detection system includes a three-axis resonant-circuit target attached to the object and a separate hand-held detection probe having a pair of parallel and coaxially aligned transmitter/sensing coils. A current is induced in the transmitter/sensing coils that determines whether a return signal strength of the target is sufficient to be counted as a valid signal. The hand-held detection probe also has a pair of receiver coils positioned within the transmitter coils and connected in a series-opposed fashion. The receiver coils allow for the creation of a null circuit condition when the target is equidistant from each of the receiver coils. The detection probe also has a visual display coupled to the receiver coils and configured to indicate the direction (e.g., left/right/up/down) in which the probe should be moved to center the detection probe over the object for achieving the null circuit condition.
Further details regarding prior systems may be found in U.S. patent application Ser. No. 10/027,675 entitled “System For Excitation Of A Leadless Miniature Marker” filed Dec. 20, 2001, U.S. patent application Ser. No. 10/044,056 entitled “System For Excitation Of A Leadless Miniature Marker” filed Jan. 11, 2002, and U.S. patent application Ser. No. 10/213,980 entitled “System For Spatially Adjustable Excitation Of Leadless Miniature Marker” filed Aug. 7, 2002.
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